Could you please help me with the last sentence in the second test? Why don't we use "is " in the second version of the sentence, I mean can we say The smaller the fruit is, the sweeter it is? Why is it omitted? Is there a rule or something?

'The smaller the fruit, the sweeter it is' is indeed the correct answer. If you click on the Finish button, that is what you should see as the correct answer. Does that help you? If not, could you please rephrase your question?

In English we often miss out certain words if they are obvious from the context and we think the sentence sounds better without them, or if including them would be repetitive. The name for this is ellipsis. You can find more information and examples of this here.

Note that it would be fine to include 'is' in the sentence, as you did in your example - that would be perfectly correct too.